In hitherto designed power plants of the above kind, any of the following feeding systems has been used:
a) Coal and absorbent are crushed and mixed. Of the crushed material a paste is prepared by the addition of water and possibly some emulsifying agent or oil which improves the transport properties of the paste. The past is pumped into the bed via nozzles. PA0 b) Coal and sulphur absorbent are crushed and dried, whereupon feeding to the bed takes place via a lock hopper system. PA0 c) Coal is crushed and fed as a paste. Absorbent is crushed and fed in dry state.
This method has certain disadvantages and limitations. To obtain good transport properties of the paste, the distribution of the particle size is very important. This places heavy demands on crushing equipment and supervision of the particle size distribution. From an economic point of view, the method can only be used for high quality coal with a low sulphur content and ash content. For coal which is rich in ash and coal with a high sulphur content, which requires a high percentage of sulphur absorbent, the water content relative to the coal quantity will be high, which has a greatly negative influence on the efficiency of the plant.
This method is used for coal with a high ash content and/or a high sulphur content. No addition of water, which reduces the efficiency, is necessary. However, energy-demanding drying of the crushed material is required to prevent moist fine-grained material from clogging lock hopper and transport systems. Dry fine-grained pulverized coal also entails an explosion hazard which may require inert gas in the lock hopper system, which greatly increases both the investment and the operating costs.
When feeding fine-grained pulverized coal in dry state, a plurality of feeding points are required to obtain a uniform temperature distribution. These feeding points must be located near each other in the lateral direction (.ltoreq.1 m between the feeding points). In addition, in order to obtain good combustion, the bed must be high (2-3 m). Otherwise there is a risk that the fine-grained coal particles blow off before they have burnt up.
If the bed is made high, also fuel nozzles must be positioned at different levels in the bed to avoid too high temperatures in the bottom zone, which would entail a risk of sintering.
As will be seen, there are considerable difficulties connected with the feeding of fine/grained coal in dry state.
All coal must be crushed into a grain size suitable for the preparation of a paste. This means that the coal must be crushed into a grain size which, on average, is small and that the crushing cost will be high.